Full understanding of the structure and function of a biological system requires a variety of structural information at a variety of levels of resolution. Rational understanding of biological systems and disease processes requires detection of the distribution, abundance, localization and function of biological molecules and processes at the cellular, subcellular and macromolecular level.
A wide variety of biological labeling and staining methods are available for detecting and localizing biological targets, each with advantages and limitations which suit it to different applications. In addition, the practice of molecular medicine depends upon accurate and reliable methods for the laboratory assessment of the presence, amount, distribution, density, and form of target analytes, both by eye using methods such as Western blotting, Southern and Northern blotting, gel staining, and immunoblotting, and microscopically using light and electron microscopy to evaluate the cellular distribution of targets such as genes by in situ hybridization, proteins by immunohistochemistry, and other molecules including but not restricted to hormones, carbohydrates, enzymes, and peptides. This is accomplished by a variety of processes that use a probe, targeted to the entity under consideration, attached to a label which then generates, either through its intrinsic properties or by specific interaction with other components applied subsequently, a signal which is detected, quantitated, or mapped to identify the presence, locations and amounts of the target.
Examples of probes include antibodies that react with a specific protein, oligonucleotides that are complementary to a genetic target, peptides that bind to specific protein subunits, or substrates that react with a target enzyme. A wide variety of methods are used for target visualization: these include fluorescent labels; particulate labels such as colloidal gold or other metals, which may be visualized directly by electron microscopy, or enlarged or rendered visible by light microscopy by other optical methods, or directly by eye through the deposition of additional metal from solution by autometallography; other intensely colored particles such as selenium or colored latex; or radionuclides that are used to expose film placed in proximity to the specimen. Enzymes targeted to the site of interest are widely used to visualize the target by treatment and reaction with a chromogenic substrate which develops color upon reaction with the enzyme. They may also be used to generate a signal indirectly, by the in situ generation of multiple copies of a target (for example, the polymerase chain reaction), the generation or deposition of multiple copies of a signal generating entity such as a fluorescently labeled substrate, or the deposition of multiple copies of a secondary target, such as biotinylated tyramide which is then visualized using a biotin-binding probe. These may be either linked to the probe directed against the target, or conjugated to a secondary, tertiary or other probe that is bound to the target in a subsequent step, either by reaction with the primary probe, or through an intermediate bridging or linking step.
Although metallographic labeling and detection, both using autometallographically enhanced gold particles and using enzyme metallography, affords greater sensitivity and specificity than many other detection methods for the optical and microscopic imaging of targets, the nature of the deposited metal means that the signal always appears as a black, completely opaque signal. If the visualization of a second overlapping target is required, visualization of this second target will be obscured by the metallographic signal. For example, enzyme metallography (EnzMet™) has achieved both ultrasensitive, high-resolution detection and localization of individual gene copies by conventional brightfield microscopy in situ hybridization, and highly sensitive and specific histochemical staining of target proteins in paraffin-embedded tissue sections. However, its application to spectral imaging and other automated imaging methods has been limited because the black signal does not give an identifiable spectrum, and therefore may be difficult to resolve from overlapping stains.
The current development and future practice of biomedical research and molecular medicine will increasingly require the detection and spatial evaluation of multiple targets simultaneously in order to study and evaluate relationships between components of a system, or to evaluate a series of different markers whose combined pattern of expression indicates the biologic behavior of the system under study or provides a prognosis for a disease process. In order to do so, methods to separate and resolve the signals used to visualize combinations of targets are required in order to accurately assess each target. This is usually achieved by the use of different colors; an example is the use of different colored fluorescent labels to assess both a target gene and a control gene, or to combine gene and protein assessment. While the human eye can resolve a small number of colors, an important advance is the use of spectroscopic image analysis (“spectral imaging”) and related methods, in which the spectral signature of each point within an image is resolved to differentiate the spectra of the dyes present, and thus enable the quantitation of overlapping multiple signals. In this way, even colors that appear similar or identical to the human eye may be resolved spectroscopically.
The combination of the increased sensitivity and resolution of metallographic detection, with the ability to detect multiple colors that is conferred by the use of colored substrates, would provide greatly improved accuracy in the evaluation of biologic and other targets. In color photography, silver is reacted with aromatic dye precursors, known as dye couplers, to produce intensely colored organic dyes. This procedure is known as dye coupling. This process has been applied to the development of colors from metallographic deposits used for the detection of biological or chemical targets on only two occasions: by Haase, in conjunction with radioisotope labeling in cultured cells, and by and Fritz, in the detection of silver-enhanced colloidal gold particles.
Correlation of information at different levels of resolution usually requires multiple labeling and detection experiments. These necessitate lengthy procedures, impose difficulties in correlating the different data sets, and may allow the structural integrity of the specimen to be compromised.
Larger gold labels pose a particular problem for correlative labeling. Their use in combined fluorescent gold probes is limited because they quench the fluorescence through resonance energy transfer; in order to preserve useful fluorescence, even 6 nm gold and fluorescent labels must be conjugated to separate antibodies. Furthermore, fluorescence microscopy is a darkfield method. Staining cannot easily be visualized in the context of tissue morphology: this can be a critical disadvantage for users such as pathologists, who much prefer to evaluate staining in the context of tissue morphology by conventional brightfield light microscopy.